Microwave heater and applicator therefor

ABSTRACT

A microwave heater for heating sheet material or material that can be carried in sheet form including an applicator having a two-dimensional microwave circuit including an array of elongate conducting elements forming a modified meander type slow wave structure supported on a quarter wave-length conducting stubs over a ground plane and transversely fed by a coaxial coupler through an aperture in the ground plane. A circuit is also provided for supplying a gas to remove vapor or moisture from material being heated.

. Q Umted States Patent 1111 3,55 40 [72] Inventors Donald A. Dunn [56] References Cited A UNITED STATES PATENTS Jurgmm 2,758,188 8/1956 Okress 219/1055 [2 p 836,809 2,937,259 5/1960 De Bell, Jr 219/1055 1 FM 2611969 3,221,132 11/1965 swam 219/1055 [45] 3,449,836 6 1969 Schreiberet al 219/1055 [73] Ass1gnee Genesys System, Inc.

Mountain View, Calif. Primary Examiner-Joseph V. Truhe I corporation of California Assistant ExaminerL. H. Bender Attgrney-Flehr, Hohbach, Test, Albritton & Herbert MICROWAVE EATER AND APPLICATOR ABSTRACT: A microwave heater for heating sheet material THEREFOR or material that can be carried in sheet form including an ap- 16 Claims Drawmg plicator having a two-dimensional microwave circuit including [52] US. Cl. 219/1055, n rr y f el ng e ing elemen s forming a modified 219/1051, 219/1019 meander type slow wave structure supported on a quarter [51] Int. Cl 1105b 9/06, a e-length conducting stubs over a ground plane and trans- I H05b 5/00 versely fed by a coaxial coupler through an aperture in the [50] Field of Search 219/ l0.55, ground plane. A circuit is also provided for supplying a gas to 10.6 I 10.79 remove vapor or moisture from material being heated.

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PATENIEU Jimzs I971 3558;840 SHEET 1 [IF 4 wumaom muse. $535.:

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D0naIdDA Dunn BY Peter .Jurg nsen WM f Attorneys INVENTOR.

. PATENTED M26 I97! SHEET 2 OF 4 L? n g. 1

Q 7* (if 3% V INVENTOR 7 00mm A. Dunn m BY Peter D. Jurgensen 44, .zM/ml; 91% 4% W Arzameys l MICROWAVE HEATER AND APPLICATOR THEREFOR BACKGROUND OF THE INVENTION This invention relates to microwave'heaters and applicators and more particularly to heaters and applicators suitable for drying operations on continuous elongate sheet material, such as photographic film or freshly printed sheet.

In the drying of photographic film, for example, there is a need for a microwave heater and applicator which can deliver a substantially uniform amount of microwave power into an entire width of film as the same is progressively moved through the applicator. Devices heretofore proposed for such applications have not been entirely satisfactory. Such devices have commonly used a microwave circuit having a maximum effective dimension of less than one wavelength in transverse extent and are accordingly limited in size by the wavelength of the electromagnetic energy used. Multiple wavelength slow wave applicator circuits have not been considered practical since devices having a circuit length greater than the wavelength have been generally found unreliable due to excitation of more than one mode resulting in an uncertainty in the power distribution and a loss of power due to radiation. A

possible applicator using a plurality of individual microwave circuits, each of less than one wavelength in transverse and grouped together to cover their combined area, has not proved practicable.

Where waveguides have been provided with slots for passing the material through the associated fields, the resultant structures are characterized by a two-sidedness which Iirnitsthe ability with which moisture or other condensable vapors can be removed. Additionally, such two-sided structures inherently provide inadequate space through which the material must pass. Where dielectrically supported transmission lines are used for these purposes, the power level is severely limited by the tendency for arc tracking and breakdown to occur across the dielectric surfaces. i

There is, therefore, a need for a new and improved microwave heater and applicator which is particularly useful in the drying of film, paper or wet material in sheet fonn or for other processes requiring the application of microwave power to a sheet material.

A general object of the invention is to provide a microwave heater and applicator therefore which can overcome the above limitations and disadvantages and which delivers a large fraction of microwave power to the material being treated in a well defined manner over a given surface area thereof.

Another object of the invention is to provide a microwave heater and applicator therefore of the-above character which provides a device capable of being constructed with dimensions much larger than the wavelength of the microwave power being used and which generates an apparent standing wave pattern in which a plurality of standing'waves are apparent over a two-dimensional region many times larger than one wavelength.

Another object of the present invention is to provide a microwave heater and applicator of the above character in which the applicator utilizes a type of slow wave structure constructed entirely of conductive elements to prevent arcing between components thereof.

Another object of the invention is to provide a microwave applicator of the above character which is designed so that a minimum amount of power is lost due to radiation of fields from the applicator.

Another object of the invention is to provide a microwave applicator of the above character designed and constructed entirely of conductive elements arranged for a direct, efiicient cooling of the applicator structure without compromising its electrical performance,

Another object of the invention is to provide a microwave applicator constructed for operation in a high passband and providing a high phase velocity to'thereby obtain a minimum rate of decay of induction fields and therefore a maximum useful fringing of fields in a direction extending away from the plane of the applicator.

The above objects of the invention are achieved in a microwave heater including generally an applicator coupled to a source of microwave power through a suitable transmission line. The applicator uses an array of elements interconnectcd by bars to a modified meander type slow wave structure supported on quarter wavelength conductive stubs over a ground plane. An important feature of the present invention resides in a topological feature in that the slow wave structure comprises a one-conductor, one-sided structure presenting fringing fields for useful work on that side away from the ground plane, where full access is available for treatment of material. As a dryer, the slow wave structure is easily incorporated in between means forming a gas flow throughout the region of the structure and about material being treated.

The disclosure herein is particularly directed to a heater using electromagnetic energy in the microwave region of the spectrum. However, it will be understood that the disclosure herein can be scaled to operate in other regions of spectrum. Accordingly, the term microwave should be regarded as specific as to the technique taught in the present invention but general as to frequency. In fact, if a large enough structure were needed, the present invention could be constructed for use at megacycle wavelengths.

These and other objects and features of the invention will become apparent from the following description and claims when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view partly in schematic of a microwave heater and applicator therefor constructed in accordance with the present invention. 1

FIG. 2 is a top plan view'of the applicator portion of the apparatus taken along the lines 2-2 of FIG. 1 with portions thereof broken away to show the details of internal construction. I

FIG. 3 is a cross-sectional view taken along the lines 3-3 of FIG. 2.

FIG. 4 is an end view taken along the lines 4-4 of FIG. 2.

FIG. 5 is a cross-sectional view taken along the lines 5-5 of FIG. 2.

FIG. 6 isa cross-sectional view taken along the lines 6-6 of FIG. 2.

FIG. 7 is a plan view of a portion of the electrical slow wave structure of the applicator of FIG. 2 emphasizing the effects produced in material being treated.

FIG. 8 is a cross-sectional view taken along the lines 8-8 of FIG. 7.

FIG. 9 is a cross-sectional view taken along the lines 9-9 of FIG. 7.

FIG. 10 is a plan view of an alternate embodiment of an electrical slow wave structure similar to that shown in FIG. 7.

FIG. 11 is a cross-sectional view of a portion of a water cooled applicator constructed in accordance with the invention.

DETAILED DESCRIPTION or THEPREFERRED EMBODIMENTS The microwave heater consists generally of a suitable source 20 of microwave power connected to an applicator 22 through a hollow rectangular waveguide 24 which can include a VSWR detector 26, such as a dual directional coupler with crystal detectors, for accurate measurement of power transfer and match between the power source and applicator. The input feed to the applicator consists of a waveguide-to-coaxial transition 28. The microwave energy remaining after the microwaves have passed through the applicator and the material being heated is removed through an output coax-towaveguide transition 32 and is absorbed in a matched load 34 which is supplied with a cooling fluid through suitable piping 36. The measurement of VSWR with detector 26 and power consumed in the load 36 (by means of a detector, not shown,

similar to detector 26) provide a direct indication of power available for delivery by the applicator.

The applicator 22 of the present invention generally includes a microwave circuit lying in a plane'surface parallel to a plane in which a sheet or other elongate material is en extended as by being passed as a web 38 under tension between rolls 40. 42 along a planar path through the applicator. Where evaporation of part of the material being treated is the objective process, a gas circuit indicated by arrows 44. 45 is made to pass through the microwave circuit and to impinge upon the material being treated to thereby remove vapors given off by the material and prevent condensation within the applicator. The gas circuit also serves to generally enhance evaporation by lowering the partial pressure of the gas phase of the materialb eing removed.

The microwave circuit consists of a slow wave structure of the ladder type which is made up of a linear array of identical circuit elements (individually numbered 464. 46-2, 46-3, 464..., 46-7...) made of conductive material and coupled together. More specifically, the slow wave structure is, in part, electrically characterized as a meander line established over a conductive ground plane 48 which isformed by one surface of a boxlike structure 50 opening downwardly away from the ground plane and closedby a flat baseplate 52 which serves as a mounting base for the applicator. The boxlike structure 50 and base plate 52 together define a lower gas plenum 54 which is served by inlets 56 and 58passed through the mounting plate. Theground plane is provided with'a plurality of substantially uniformly distributed small apertures 60 therein for permitting the flow of gas from below the ground plane to the inside of the applicator. The flow of gas throughthe aperture creates upwardly. directed air streamor jets which-typically have sufficiently high velocity to, cause stripping of the.

vaporized solvents or moisture fromthe bottom of the material beingtrcated. Further, the air jets serve to provide positive direct cooling of the resonator elements.

Referring particularly to FIGS. 2 through 9, the structure of the microwave circuit of the applicator of the present invention is shown in greater detail. Each of the resonator elements 46 consists of a conductive rod of circular section having a step 47 of reduced diameter at each end. The length of the rod between the steps 47 of reduced diameter is the same for each element. The rods are arranged in spacedparallel relation to each other by a pair of side rails 62, 64. which are provided with a plurality of spaced holes 66 which receive the respective steps 47 of the'elements and serve, to support them in spaced relation above the ground plane 48.

As shown in FIGS. 6 and 9, coaxialtransition 28 includes an input terminal including a post 68 extending transversely through the ground plane through aperture 69 and connected by a screw 70 to the first element 46-1 to define an input feed location 73. The lower end of the post' 68 fits within a coaxially disposed elongate passageway 72 in a center conductor extension 74 which is provided with a ,quarter wavelength matching step 76 at its upper end and a bullet 78 at its lower end for interfit ting with a mating connector (not shown) of the center conductor of the coaxial transition 28-to facilitate astion and therail serving as a quarter wavelength stub which has a'hi'gh impedance at the wavelength of operation.

Means are provided for forming the Iadderlike array of ele ments 46 'into' a slow wave structure including a modified meander line and consists of a plurality of conductive interconnecting bars 86a, 86b which electrically bridge between adjacent ones of elements. The first interconnecting bar 86b-1 is positioned adjacent the opposite end from the input feed location 73 and serves to connect the first and second elements 46-1, 46-2 between positions 46-112, 46-212 approximately one-quarter wavelength from the adjacent support rail 64.

The second and third elements 46-2, 463 are similarly bridged together adjacent the other side ofthe structure by an interconnecting bar 86a l connecting them at positions 46-2a. 46-30 and is approximately one-quarter wavelength from the adjacent rail. This pattern is repeated to form a single conductive path including a meander structure in which each element 46 is connected with one adjacent element at one side of the array and to the other adjacent element at the opposite side of the array to form a regular pattern having a single zig zag path in the form of a repeating S.

FIG. 8 shows the cpnstructional detail of a typical connection between adjacent elements 46 and an interconnection bar 86b. Thus, each bar 86 can corisist of a short section of tubing shaped at each end to conform to the transverse cylindrical contour of the element to which it attaches. One of the elements is provided with a transversely'directed hole 90 which receives a pin 92 slidably disposcdin thebar. The hole 90 is accurately positioned to obtain thef correct length for the ac tivelength of the element and f or the remaining stub.

After assembly of all ba'rs86,'the entire array takes the form of a meander line structure supported by'the end portions 'of the elements which serve as quarter'wavelength conductive stubs. r j

A second gas inlet plenum-is formed above the slow wave' structure and'consist's of a boxlike structure 95 having air inlets 96, 98 through its upperside and closedon its Iowerside by a perforated plate 100 which .lies generally parallel to and spaced from the elements 46. The structure 95 is supported by any. suitablemeans, such as by downwardlyextendingaprons I02, 104 depending fromits sides which terminate in,outwardlyextending flanges I06, I08 resting upon the rails'62 64 and secured in place as with screws I09 passed through the edges of the ground plane 48. Downwardlyfextendin g-aprons H0; 112 are also provided at the material inlet and outlet sides. and terminate in spaced relation to the slow wave circuit to provide access space [I3 for material to movethrough the applicator. Gas passed through the upper plenum serves to carry away vapors emitted or driven from the material being heated. Both: upper and lower plenu'msare usually supplied with air under pressurewhich is exhausted through ports I15 formed in aprons 102,104. Structure and associated apronsalso serve as a radiation shield for preventing radiation from escaping from the applicator.

The. operation of the applicator and the heater of thepresent invention can best be understood from reference to FIGS. 7 and 10. FIG. 10 shows a meander line which is the electrical equivalent and an alternate embodiment of the line of FIGS. I through 9" but is developed in a different manner. Thus. the circuitof FIG/l0 includes a plurality of conductive elements l46inte rconnected at alternate ends supported on quarter wavelength 'stubseonnectingthe midpoint of the interconn ection bars andthe railorground; (Similar parts have been given like numbers raised by the addition of I00.) Ineither case, the electrical result is similar. The quarter wavelength stubs do. not short out the structure for the frequencies of operation, and serve to support the slow wave line without the necessity of providing conventional insulating support structure with its potential for arcing.

The apparent standing wave patterns of high intensity, fringing field de'veloped'in 'the. present structure are depicted by the" oval regions which are interposed between the elements "wave structure proper. Although these regions-122 are atstructure at the position of the material being heated, by the input impedance that the structure presents to the source and by the circuit losses both ohmic and radiated. More specificaily, the diameter of the elements should be made comparable to the spacing between them since, if made smaller, it

tends to increase the input impedance and the circuit attenuation; and, if made larger, it not only decreases the input impedance but tends to defeat adequate fringing of the fields whereby the fields extend appreciably above the structure and into the region occupied by the material to be heated.

The preferred interelement spacing or pitch is determined to be about one-eighth of a wavelength. If the spacing is increased much above this, the structure begins to radiate execs sively. The pitch is also determined by the desired phase velocity down the slow wave structure. This velocity is found to lie in the range of about 0.5 for apitch of about one-eighth of a wavelength and less than one-quarter wavelength.

To maximize interaction impedance, the spacing between each element centerline and the ground plane should be made as large as possible without destroying the input impedance match or causing excessive radiation. Spacing of about onesixteenth wavelength has been found satisfactory.

The following relates to an applicator which was constructed for operation at 2,450 megacy'cles. This applicator 'utilized a pitch distance of approximately one-eighth of a wavelength, an element diameter of about one-quarter inch, and an element centerline to ground plane distance of one-sixteenth wavelength. The apparent standing wave pattern is shown encircled by the dotted oval patterns 120 and 122 of FIG. 7. This also corresponds to the pattern of heating in the material being treated. The applicator was found to operate in the fifth passband above the lowest passband.

ln the present disclosure, the lengths of the elements, stubs, and spacing of the components are defined in terms of the wavelengths of the center of the passband of preferred operation. It is to be understood, however, that the applicator structure remains operative overv a wide range of frequencies not only within the design passband, but also at other passbands. By way of example, use of another passband having a percent difference in frequency has been found to yield satisfactory results. Accordingly, the specification of particular lengths, such as in quarter wavelengths, is intended to be generally descriptive at a center frequency of a principal passband and is not intended to exclude operation in adjacent passbandsat which the heater may also be operable.

By providing an applicator and microwave heater whichis completely conductive in design, it is possible to obtain numerous advantages in its construction. All of the materials are conveniently made of conductive metal. The elements can be copper coated steel or aluminum or, in the case of tubing, can be of a copper-base rigid alloy. All of the structure of the slow wave circuit proper is firmly and solidly supported on rigid metal rails from which no insulation subject to breakdown is required. The structure lends itself to adequate cooling either by making the elements of tubing and passing a fluid through them and/or by the direct cooling action of gas flow upwardly and around the elements before contacting the material being heated. The width of the slow waves structure can be made with any odd multiple of one-quarter wavelength, the threequarter wavelength case (between rails) being the minimum possible before the structure becomes degenerate.

By utilizing the structure of the present invention, an additional advantage is achieved since the upper edge of the elements can be made to be flush with the side rails. This permits the material being dried to be passed over the applicator slow wave circuit at an angle to the rails and to pass over the rails in arbitrarily close proximity. The angle of passage can be selected to improve uniformity of the heating coverage and thereby avoid striping. The applicator shown in FIGS. 1 through 9 would have to be modified in order to do this but the modification is straightforward and only involves utilizing another suitable means for supporting the upper air plenum so as to eliminate at least portions of the aprons 102, 104.

Additionally, it should be pointed out that the material being passed through the applicator need not be exactly in a plane parallel with the slow wave structure but can be positioned at an angle so that the one side of material rises away from this plane as it passes through the applicator. By using this feature, it is possible to control the amount of power delivered to regions where the material is wet, for example, than where it is dry. This is useful since the microwave coupling is more efficient where the moisture content is high. Thus, material could be fed counter to the flow of microwave power in the device to overcome any difficulty in getting sufficient power into the drier portion of the material and thus obtain a more uniform drying action.

The present invention is particularly adapted for direct cooling of the slow wave structure. Thus, as shown in FIG. 11, rail 262 can be provided with a passageway therein for receiving coolant and communicating with a passageway formed in each of the elements 246. Similarly, rail264 is provided with a passageway which serves as a spent coolant drain. Obviously, the elements can be constructed of conductive tubing. in high power applications, such direct cooling is very useful to prevent the elements from distorting under heat. With the present structure, the direct supply of coolant does not appreciably interfere with the electrical operation of the applicator.

We claim:

1. In an applicator for microwave heating, means forming a ground plane lying on a two-dimensional surface,'a plurality of elongate conductive elements arranged in a parallel array and spaced from said two-dimensional surface, a plurality of conductor bar interconnecting adjacent portions of said elements to form a conductively continuous structure such that each element is connected with the one adjacent element at one side of the array and to the other adjacent element at its other side in a regular pattern forming a singleconductive path, a plurality of substantially quarter wavelength conductive stubs, each of said stubs being connected between respective ones of said conductor means and said ground plane said quarter wavelength stubs including two conductive arms each consisting of an extension of the respective elements beyond each interposed conductor bars and interconnecting the same to said ground plane.

2. An applicator as in claim 1 in which each of said elements is tubular and means connecting one end of each of said elements to a source of coolant fluid and the other end to a spent coolant drain.

3. An applicator as in claim 1 wherein said elements have a circular cross section.

4. An applicator as in claim I wherein each of said elongate elements has an electrical length corresponding to odd multiples of quarter wavelengths at the midband of operation and greater than three-quarters of a wavelength.

5. In an applicator for microwave heating, means forming a ground plane lying on a two-dimensional surface, a plurality of elongate conductive elements arranged in a parallel array and spaced from said two-dimensional surface, a plurality of conductor bar interconnecting adjacent portions of said elements to fonn a conductively continuous structure such that each element is connected with the one adjacent element at one side of the array and to the other adjacent element at itsother side in a regular pattern forming a single conductive path, a plurality of substantially quarter wavelength conductive stubs, each of said stubs being connected between respective ones of said conductor means and said ground plane said quarter wavelength stubs including single conductors connected to the midpoint of each of said conductor bars interconnecting a pair of adjacent elements and interconnecting the same to the ground plane.

6. An applicator as in claim Sin which each of said elements is tubular and means connecting one end of each of said elements to a source of coolant fluid and the other end to a spent coolant drain.

7. An applicator as in claim wherein said elements have a circular cross section.

8. An applicator as in claim 5 wherein each of said elongate elements'has an electrical length corresponding to odd multiples of quarter wavelengths at the midband of operation and greater than three-quarters of a wavelength.

7 9. in an applicator for microwave heating, means forming a ground plane lying on a twodimensional surface, a plurality of elongate conductive elements arranged in a parallel array and spacedfrom said two-dimensional surface, a plurality of conductor bars interconnecting adjacent portions of said elements to' form a conductively continuous structure such that each element is connected with the one adjacent element at one side of the array and to the other adjacent element at its other side in a regular pattern forming a single conductive path, a plurality of substantially quarter wavelength conductive stubs, each of said stubs being connected between respective ones of said conductor means and said ground plane, a pair of conductive side rails positioned on each side of said array and transversely of the elements thereof, each of the quarter wave stubs being connected to terminate on a respective one of said rails.

10. An applicator as in claim 9 wherein said rails terminate at the surfaces of the elements remote from said ground plane to thereby lie on a surface flush withsuch surfaces.

11. A microwave heater comprising a source of microwave power, an applicator for microwave. heating comprising means forrninga ground plane lying on a two-dimensional surface, a plurality of elongate conductive elements arranged in a parallel array and spaced from said two-dimensional surface, a plurality of conductor bars for interconnecting adjacent portions of said elements to form a continuousconductive structure such that'each element is connected-with the one adjacent element at one side of the array and to the other adjacent element at its other side in a regular pattern forming a single conductive path, a plurality ofquarter wavelength conductive stubs, each of said stubs being connected between respective ones of said conductor means and said ground plane, transmission line connecting said source to said applicator. the end of said transmission line adjacent said applicator including at least two conductors, one of said conductors being electrically connected to an element of said array, and the other of said conductors being electrically connected to said ground plane.

12. A microwave heater as in claim ll in which said trans mission line adjacent said applicator comprises means forming a coaxial transition having cylindrical outer conductor and a central conductor coaxially mounted within said outer conductor, means electrically interconnecting a conductor to a first active element of said array.

13. A microwave heater as in claim 12 further including means for mounting said coaxial transition transversely to said ground plane on'that side away from said slow wave structure with said outer member in electrical contact with said ground plane, said ground plane having an aperture therein through which said central element extends, said aperture and said coaxial transition being positioned immediatelyv adjacent a first active element of said array, whereby said array is transversely fed by said coaxial transition through said. ground plane.

14. An applicator as in claim 9 in which each of said elements is tubular and means connecting one end of each of said elements to a source of coolant fluid and the other end to'a spend coolant drain.

15. An applicator as in claim 9 wherein said elements have a circular cross section. i

16. An a'ppltcatoras in claim 9 wherein each of said elonga'te elements an electrical length corresponding to odd multiples of quarter wavelengths at the'midband of operation and greater than three-quarters of a wavelength. 

1. In an applicator for microwave heating, means forming a ground plane lying on a two-dimensional surface, a plurality of elongate conductive elements arranged in a parallel array and spaced from said two-dimensional surface, a plurality of conductor bar interconnecting adjacent portions of said elements to form a conductively continuous structure such that each element is connected with the one adjacent element at one side of the array and to the other adjacent element at its other side in a regular pattern forming a single conductive path, a plurality of substantially quarter wavelength conductive stubs, each of said stubs being connected between respective ones of said conductor means and said ground plane said quarter wavelength stubs including two conductive arms each consisting of an extension of the respective elements beyond each interposed conductor bars and interconnecting the same to said ground plane.
 2. An applicator as in claim 1 in which each of said elements is tubular and means connecting one end of each of said elements to a source of coolant fluid and the other end to a spent coolant drain.
 3. An applicator as in claim 1 wherein said elements have a circular cross section.
 4. An applicator as in claim 1 wherein each of said elongate elements has an electrical length corresponding to odd multiples of quarter wavelengths at the midband of operation and greater than three-quarters of a wavelength.
 5. In an applicator for microwave heating, means forming a ground plane lying on a two-dimensional surface, a plurality of elongate conductive elements arranged in a parallel array and spaced from said two-dimensional surface, a plurality of conductor bar interconnecting adjacent portions of said elements to form a conductively continuous structure such that each element is connected with the one adjacent element at one side of the array and to the other adjacent element at its other side in a regular pattern forming a single conductive path, a plurality of substantially quarter wavelength conductive stubs, each of said stubs being connected between respective ones of said conductor means and said ground plane said quarter wavelength stubs including single conductors connected to the midpoint of each of said conductor bars interconnecting a pair of adjacent elements and interconnecting the same to the ground plane.
 6. An applicator as in claim 5 in which each of said elements is tubular and means connecting one end of each of said elements to a source of coolant fluid and the other end to a spent coolant drain.
 7. An applicator as in claim 5 wherein said elements have a circular cross section.
 8. An applicator as in claim 5 wherein each of said elongate elements has an electrical length corresponding to odd multiples of quarter wavelengths at the midband of operation and greater than three-quarters of a wavelength.
 9. In an applicator for microwave heating, means forming a ground plane lying on a two-dimensional surface, a plurality of elongate conductive elements arranged in a parallel array and spaced from said two-dimensional surface, a plurality of conductor bars interconnecting adjacent portions of said elements to form a conductively continuous structure such that each element is connected with the one adjacent element at one side of the array and to the other adjacent element at its other side in a regular pattern forming a single conductive path, a plurality of substantially quarter wavelength conductive stubs, each of said stubs being connected between respective ones of said conductor means and said ground plane, a pair of conductive side rails positioned on each side of said array and transversely of the elements thereof, each of the quarter wave stubs being connected to terminate on a respective one of said rails.
 10. An applicator as in claim 9 wherein said rails terminate At the surfaces of the elements remote from said ground plane to thereby lie on a surface flush with such surfaces.
 11. A microwave heater comprising a source of microwave power, an applicator for microwave heating comprising means forming a ground plane lying on a two-dimensional surface, a plurality of elongate conductive elements arranged in a parallel array and spaced from said two-dimensional surface, a plurality of conductor bars for interconnecting adjacent portions of said elements to form a continuous conductive structure such that each element is connected with the one adjacent element at one side of the array and to the other adjacent element at its other side in a regular pattern forming a single conductive path, a plurality of quarter wavelength conductive stubs, each of said stubs being connected between respective ones of said conductor means and said ground plane, transmission line connecting said source to said applicator, the end of said transmission line adjacent said applicator including at least two conductors, one of said conductors being electrically connected to an element of said array, and the other of said conductors being electrically connected to said ground plane.
 12. A microwave heater as in claim 11 in which said transmission line adjacent said applicator comprises means forming a coaxial transition having cylindrical outer conductor and a central conductor coaxially mounted within said outer conductor, means electrically interconnecting a conductor to a first active element of said array.
 13. A microwave heater as in claim 12 further including means for mounting said coaxial transition transversely to said ground plane on that side away from said slow wave structure with said outer member in electrical contact with said ground plane, said ground plane having an aperture therein through which said central element extends, said aperture and said coaxial transition being positioned immediately adjacent a first active element of said array, whereby said array is transversely fed by said coaxial transition through said ground plane.
 14. An applicator as in claim 9 in which each of said elements is tubular and means connecting one end of each of said elements to a source of coolant fluid and the other end to a spend coolant drain.
 15. An applicator as in claim 9 wherein said elements have a circular cross section.
 16. An applicator as in claim 9 wherein each of said elongate elements has an electrical length corresponding to odd multiples of quarter wavelengths at the midband of operation and greater than three-quarters of a wavelength. 